M1078A1 cab and chassis electrical upgrades including AC

[aw113sgte]

Sorry for your troubles! I don't trust manufacturers specs unless they are major companies with technical details to back it up. I have a 9000 BTU Fujitsu mini split for a 20ft shipping container and it's been able to keep it to my set temp under all conditions we've been in (up to 100f humid, down to ~15f) 2" spray foam on all walls/ceiling/floor.
I have a different battery setup 15kwh lifepo4. Run the AC/heat 24/7. Usually down to 65-75% after one day before solar starts recharging.
We are piping the AC from the habitat to the cab during driving, works okay, need to add some tint to the window

 

[hike]

Thank you.

Not sure our troubles are with the AC. We are struggling more with the math and understanding how to check the specifications when moving from watts to BTU's. We are accomplishing at least 15° differential between inside and outside temperatures now that we removed the Victron combiner and we have yet to finish the insulation and add the carpeted trim pieces. The AC is likely doing what it is supposed to do and will only be more effective as we complete more.

The question has become more about checking what manufacturers put in their spec sheets. Red Dot makes a similar roof top 24v AC they state is rated for 16,000 BTU's cooling power. The spec sheet goes on to say at 24v it draws 94.2a or 2,260 watts at 100° or 111a at 110°. If there are 3.412 BTU/hour in a watt (typically watt/second) then 2260 (94.2 * 24) watts is only 7,714 BTU's not, 16,000. Red Dot knows how to calculate BTU's better than I do. I get similar results with the numbers for our unit.

What am I missing? (edit) The correct formula it turns out. Thank you Ronmar. (/edit)

We also need to do tint, maybe ceramic? The radiant energy to surfaces is always more than AC can overcome—

 

[Ronmar]

Thank you.

Not sure our troubles are with the AC. We are struggling more with the math and understanding how to check the specifications when moving from watts to BTU's. We are accomplishing at least 15° differential between inside and outside temperatures now that we removed the Victron combiner and we have yet to finish the insulation and add the carpeted trim pieces. The AC is likely doing what it is supposed to do and will only be more effective as we complete more.

The question has become more about checking what manufacturers put in their spec sheets. Red Dot makes a similar roof top 24v AC they state is rated for 16,000 BTU's cooling power. The spec sheet goes on to say at 24v it draws 94.2a or 2,260 watts at 100° or 111a at 110°. If there are 3.412 BTU/hour in a watt (typically watt/second) then 2260 (94.2 * 24) watts is only 7,714 BTU's not, 16,000. Red Dot knows how to calculate BTU's better than I do. I get similar results with the numbers for our unit.

What am I missing?

We also need to do tint, maybe ceramic? The radiant energy to surfaces is always more than AC can overcome—

I think the detail you are missing is that the watt to BTU conversion(3412BTU/KW/HR) is ONLY applicable when you are CREATING heat directly from electricity. IE: A 1KW/HR heating element will deliver 3412BTU/HR of heat into air or water… refrigeration or Air Conditioning is not “creating” heat, it is collecting, concentrating and transferring heat. Because of this, heat pumps can be many times more efficient at heating a dwelling that straight electric heat.

A/C is typically rated in tons and not BTU, and the BTU may have been adopted to help people compare it to the effectiveness of conventional heaters.

In order to measure actual BTU output you need to measure airflow thru the Evaporator(CFM, CFH), and the difference in input to output temp thru the evap(should be at least in the low teens, 13+ degrees temp drop)… Like 1 BTU will heat 1# of water 1 degreeF, 1 BTU will heat or cool X amount of air 1F. Forget the conversion right off the top of my head, google it That’s how you determine actual A/C BTU output…

 

[Ronmar]

Ok, here is a calculator for you. Add in any two fields(CFM and deltaT) and it will solve for the third. Such as 1000CFM @ 13F temp drop,is 14,040BTU.

To measure flow you will need something to measure the air velocity. I have a small handheld weather station with a built in anemometer that measures velocity in MPH or FPM. It can also do the flow measurement directly by entering the duct dimensions. Duct area in sq/ft X velocity in Feet/min = cu/ft per min…

CFM and BTU Calculator

 

[hike]

I found a similar formula: CFM = BTU / ( dT x 1.08 ) or BTU = CFM x dT x 1.08

Rated flow in Max is 2,423fpm, or 400cfm (12 x 2 vent to four 3" diameter vents, so let use 350 likely actual)
Observed dT is 28° between inlet and exit

350cfm x 28° x 1.08 = 10,584 BTU
400cfm x 28° x 1.08 = 12,096 BTU

Thank you @Ronmar You are mystic.

AC cools cab to about 20° below outside in our real world experience. BTU's calculate similar to manufacturer's specs using manufacturer's air flow, (testing will confirm). Electrical system works as intended to save truck batteries if AC runs while truck does not. And my use of the wrong formula for BTU calculation stirred the pot for a few posts.

As always you contribute to getting on the right path forward—

 

[Ronmar]

Comparing actual BTU to electric power used is how they determine efficiency. In A/C it is mostly a factor of outside air temp and size and airflow of the condenser…

28F is a pretty large delta for an automotive A/C. you might could improve performance a little if you can find a way to cram some more air thru the evap. like insulation delays heat gain, added airflow can increases heat absorption speed from inside the cabin. Of course the trick is to absorb it faster than it can come in, which is where insulation pays it's biggest dividend...